The continually increasing combustion of fossil fuel, such as coal, natural gas and oil, during the last centuries has resulted in an increase in the concentration of CO2 in the atmosphere. The increasing concentration of CO2 has caused concern due to the greenhouse effect caused by CO2. The greenhouse effect is suspected already to have caused at least some of the changes in the climate that have been seen during the last decades, and is according to simulation models suspected to cause even more and potentially dramatic changes in the climate of planet earth.
This has caused a call for action from scientists, environmentalists and politicians throughout the world, to stabilize or even reduce the discharge of CO2 from combustion of fossil fuel into the atmosphere. This may be achieved by capturing and safe depositing of CO2 from the exhaust gas from thermal power plants and other plants where fossil fuel is combusted.
The captured CO2 may be injected in sub terrain formations such as oil wells as pressure support for enhanced oil recovery or in depleted oil and gas wells for deposition. Tests indicate that CO2 remains in the sub terrain formation for thousands of years and is not released into the atmosphere.
In prior art, capturing of CO2 from a gas by means of absorption is well known and has been used for decades, e.g. for removal of CO2 (and other acid gases) from produced natural gas at gas fields. The absorbents used or suggested in the prior art have been different aqueous alkaline solutions, such as potassium carbonate, see e.g. U.S. Pat. No. 5,528,811, and different amines, see e.g. U.S. Pat. No. 4,112,051, U.S. Pat. No. 4,397,660 and U.S. Pat. No. 5,061,465. Separation of CO2 from exhaust gas from thermal power plants by means of an amine solution, is know e.g. from U.S. Pat. No. 4,942,734.
Common for these CO2 capturing solutions is that the gas mixture to be separated is introduced countercurrent to the aqueous absorbent in an absorber column. The gas leaving the absorber column is CO2 depleted (or acid gas depleted), whereas the CO2 (or other acid gas) leaves the absorber column together with the absorbent. The absorbent is regenerated in a regenerator column and returned to the absorber column. Amine is regenerated by stripping the amine solution with steam in the regeneration column. The steam is generated by a reboiler at the base of the column.
Solid sorbents serve as alternatives to wet chemical absorbtion via the formation of carbamate species. However, since only the surface is involved in the reaction, the quantity of CO2-reactive material that can be incorporated in the solid sorbent is limited by the specific surface area of the solid. In prior art, this severely restricts the amount of gases such as CO2 that can be absorbed by the sorbents and gives rise to short breakthrough times.
Numerous solid sorbents for CO2 removal have been developed over the years. O. Leal, et al., Inorganica Chimica Acta, 240, 183-189, 1995 have surface modified a silica gel using 3-aminopropyltriethoxysilane as the chemical moiety. The amine groups present at the solid surface after modification facilitates the CO2 adsorption via formation of carbamate species.
U.S. Pat. No. 5,087,597 awarded to Leal, et al. on Feb. 11, 1992 discloses a method for the chemisorption of CO2 at room temperature using a silica gel having a surface area of between 120 and 240 m2/g. The gel has been modified with a polyalcoxisilane containing one or more amino moieties in its structure.
U.S. Pat. No. 4,810,266 awarded to Zinnen et al. on Mar. 7, 1989 discloses a method for CO2 removal using animated carbon molecular sieves that have been treated with alcohol amines.
There is still a need in prior art for solid phase improvement as well as the process accompanied with use of solid adsorption. The success of such technology is strongly dependent of good temperature durability, material strength, high adsorption capacity and reaction kinetics and good selectivity.